The chief objective of this investigation is to clarify the mechaniism of action of cardiac cytochrome oxidase. The interrelationships of cytochromes a and a3 and copper in the enzyme will be studied in heart muscle using spectophotometric and electron paramagnetic resonance (EPR) techniques. An oxidase preparation depleted of EPR-visible copper or of total copper will be used for these studies. A cytochrome oxidase from Pseudomonas aeruginosa will be studied as a model for the cardiac enzyme. Research will include crystal structure, modification of the heme groups, amino acid sequence, and kinetics of oxygen reduction.